1. Technical Field
The present invention relates generally to fiber reinforced structural moldings such as heavy truck wheels, and more specifically to techniques for improving the strength and appearance of the moldings and for reducing molding time.
2. Description of the Related Art
Composite structures made with plastic resin are typically reinforced with chopped or continuous fibers. Greater strength and stiffness result when the fibers are straight and continuous. Exceptional properties result when plies of unidirectional fiber are bonded in layers, each in a different direction. Similar, approximately isotropic, properties result when cloth reinforcement is used.
Heavy truck wheels have been made of fiberglass, and they can have much greater strength than steel wheels. These fiberglass wheels have used sheet molding compound (SMC) having random fiber orientation. More complicated structures including various radial, axial and circumferential unidirectional fiber layers are described in McDougall U.S. Pat. No. 4,749,235. Commercial manufacture of fiberglass truck wheels, however, has not occurred. This is probably due to the cost of capital investment, labor and materials in the manufacturing process.
Various molding processes have been used for manufacturing fiber reinforced flywheels. In this case the complications of the manufacturing process are justified by the absence of alternative materials having the desired characteristics of low weight and high strength. In Kulkarni et al. U.S. Pat. No. 4,537,091, for example, it is said that some prior art flywheels have been made by the vacuum-bag/press-molding method. See Col. 3, lines 12 to 26. This method is said to consist of forming directionally ordered stacks of plies of uncured unidirectional composites. The stacks are vacuum-bag compacted and combined into layups. Each layup is put into a press and cured under vacuum at elevated temperatures. A flywheel is formed from the layup by removing it from the press and machining it as necessary.
In Col. 4, lines 54-64, Kulkarni et al. further describe a matched-die molding process for molding parts of SMC available commercially from Owens-Corning Fiberglass Corporation. The SMC sheet is formed by compacting high strength fiberglass roving and resin/filler paste between layers of carrier film. After maturation in a controlled environment, the carrier film is removed from the SMC. Then plies cut from the sheet are positioned upon one another to form a charge for placement in an appropriate mold. A part is formed from the charge when the mold is closed and subjected to the proper temperature and pressure in a matched-die molding process.
A reinforced composite flywheel made of stacked cross-ply layers is disclosed in Hatch U.S. Pat. No. 4,207,778.
A flywheel having fibers wound circumferentially about a rim is disclosed in Davis et al. U.S. Pat. No. 4,660,435. The circumferentially wound fibers are enclosed by a shell formed by fibers wound in a plurality of overlapping segments, the general direction of the fibers in each segment being tangential to the rim surface.